This invention relates to the field of prosthetics, and more particularly, to an intervertebral disc prosthesis designed to replace a damaged intervertebral disc.
The human spine consists of twenty-four small bones known as vertebrae that protect the spinal cord and provide stability to the torso. The vertebrae are arranged in a column and stacked vertically upon each other. Each vertebra is comprised of two parts including an anterior part and a posterior part. The anterior part is often referred to as the vertebral body, and the posterior part is often referred to as the vertebral arch. The vertebral bodies are generally separated by a fibrous bundle of tissue called an intervertebral disc. These intervertebral discs act as a cushion to the spinal column by absorbing energy and transmitting loads associated with everyday movement. They also prevent the vertebrae from rubbing against each other. The combination of an intervertebral disc and its associated superior vertebra and inferior vertebra may be referred to as a functional segmental unit.
Over time, the normal aging process causes the intervertebral discs to degenerate, diminishing their water content and thereby reducing their ability to properly absorb the impact associated with spinal movements. Diminished water content in the intervertebral discs may also cause the vertebrae to move closer together. Tears and scar tissue can weaken the discs, resulting in injury. When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity.
The condition of degenerative disc disease can potentially be relieved by a surgical procedure called artificial disc replacement or total disc replacement. In this procedure, the damaged intervertebral disc is replaced by an intervertebral prosthetic device (i.e., an “artificial disc” or “intervertebral disc prosthesis”). A typical prior art artificial disc comprises two endplates. One endplate faces a superior vertebra and the other endplate faces an inferior vertebra. A bearing surface is provided between the two endplates, allowing the endplates to rotate relative to one another and generally mimic the motion allowed by a natural disc.
In order to be safe and efficacious, a total disc replacement should not disrupt the normal kinematics of the functional segmental unit. Preferably, an intervertebral disc prosthesis should allow the vertebra to move with respect each other by means of the same, normal, physiological, instantaneous centers of rotation as the intact functional segmental unit. Under normal conditions a superior vertebra and inferior vertebra for a given functional segmental unit will rotate with respect to each other about differing centers of rotation, depending upon the type of movement, whether flexion/extension, lateral bending, or axial rotation (also sometimes called as “torsional rotation”). Therefore, the center of rotation that defines one type of movement may be different from the center of rotation that defines another type of movement.
As noted above, the locations of instantaneous centers of rotation in the lower cervical spine depend on the type of forces exerted on the spinal segments. While the locations of these instantaneous centers of rotation have not been precisely pinpointed, general observations have been made. For example, with respect to the C5-C6 cervical functional unit, axial rotation appears to be defined by a center of rotation (COR) that is located within the disc space when viewed from above but either to the left or to the right of the geometric center of the disc, depending on the direction of motion. For flexion/extension (F/E) movement, the COR at C5-C6 is located at the anterior portion of the subsequent, lower vertebra. In lateral bending, there is much speculation about the region of interest for determining the instantaneous centers of rotation. However, it appears that lateral bending is defined by a COR near the geometric center of the disc, which varies depending on the direction of motion.
Contemporary cervical intervertebral prosthetic devices typically have a COR that is centrally located immediately below a single contacting surface radius. While this may allow relatively normal lateral bending and axial rotation, such a design does not take into account the normal flexion-extension COR located at the anterior portion of the subsequent, lower vertebra. As far as flexion-extension is concerned, a central COR is not consistent with the normal kinematics of the functional segmental unit. Accordingly, a central COR for flexion/extension movements will force the vertebrae along non-physiologic paths.
As a vertebra goes through its ranges of motion, the pattern of motion is determined by a combination of the physical contact between the geometric anatomy of the structures, their physical properties, and the properties of the passive restraints that hold the functional units together (ligaments, muscles, etc.). The facets are the anatomical features that contribute most to dictating where the center of motion will be located. In the C5-C6 region, the facets are positioned approximately at a 45 degree angle to the disc. This is an important clue as to an anterior location for the flexion/extension COR. Thus, the centrally located COR typically found in current prosthetic discs is inconsistent with the plane of the contacting facets at the C5-C6 level.
Accordingly, it would be advantageous to provide a total disc replacement that performs consistent with the normal kinematics of the functional segmental unit. Furthermore, it would be advantageous to provide an intervertebral disc prosthesis configured to provide multiple, adaptative centers of rotation, depending upon the type of vertebral movement facilitated by the disc, including one center of rotation substantially removed from another center of rotation.